The present invention relates generally to the selective deposition of metal layers and more specifically to the selective deposition of refractory metals on semiconductor substrates.
The ability to selectively deposit tungsten on silicon exposed through openings in silicon dioxide as well as other glass surfaces is well known. The use of gases for example, tungsten hexafluoride to react with the exposed silicon to begin the deposition of tungsten on silicon is well known. Since the tungsten hexafluoride does not react with the silicon dioxide, there is no deposition on the silicon dioxide. The general temperatures range is from 500.degree.-800.degree. C. It is well known that this system will result only in a limited thickness of tungsten which can be formed on the silicon. In order to increase the thickness as well as to provide a tungsten layer on silicon dioxide, it has been suggested to introduce hydrogen with the tungsten hexafluoride. The hydrogen reacts with the tungsten hexafluoride and causes deposition onto the already formed tungsten as well as onto the silicon dioxide.
Patents which describe these methods of chemical vapor deposition are Amick, U.S. Pat. No. 3,477,872 and Tarng et al., U.S. Pat. Nos. 4,349,408 and 4,404,235. Also, an article entitled "A Planar Metallization Process - Its Application to Tri-level Aluminum Interconnects" by Moriya et al., pages 550-552 IEDM 83, IEEE also describes the selective chemical vapor deposition of tungsten from tungsten hexafluoride.
Although the prior art discusses the ability to selectively deposit tungsten on a silicon-silicon dioxide wafer, these techniques have not been used in production. As indicated in the Moriya et al. paper, after a given amount of time the selectivity disappears. The theory of operation suggested in this paper is that nucleation sites appear later in the CVD process and that with possible appropriate pretreatment these nucleation sites can be minimized.
Another problem in the prior art is the inability without substantial preprocessing of even beginning the chemical vapor deposition of metals, for example, tungsten, in a cleaned tube. The prior art generally bakes and pumps a cleaned tube at 800.degree. C. for one to two hours to generally remove between 90 and 95% of the humidity in the tube. This long drying period has not guaranteed that the tungsten or other metals will begin depositing from the chemical vapors.
Thus, it is an object of the present invention to provide a method to selectively deposit metals in a chemical vapor deposition process with a greater degree of assurance.
Another object of the present invention is to provide a process which is capable of producing a substantially unlimited thickness of metal selectively onto a substrate by chemical vapor deposition.
Still another object of the present invention is to provide a method for assuring the chemical vapor deposition of metal in a cleaned furnace.
These and other objects of the invention are attained by covering a substrate with a moisture adsorbing mask, introducing moisture onto the moisture adsorbing mask and performing a chemical vapor deposition of a gas of the metal to be deposited selectively in openings in the moisture adsorbing mask selectively as long as the moisture is present in the moisture adsorbing mask. For thicker layers of deposited metal, the process is terminated before the moisture is depleted from the moisture adsorbing mask and additional moisture is provided to be adsorbed by the moisture adsorbing mask and the chemical vapor deposition process is reinitiated. The gas flow having the gaseous metal includes a reduction gas and the flow rate is sufficient to minimize the etching from the reduced product. The process is carried out in a low pressure chemical deposition furnace at a low temperature range between 250-400.degree. C. The moisture may be added to the moisture adsorbing mask by introducing a moisture laden atmosphere and then removing it so as to remove all moisture not adsorbed by the moisture adsorbing mask from the atmosphere. Alternatively, a moisture adsorbing material may be placed adjacent to the moisture adsorbing surface to provide a deposition inhibiting barrier at moisture adsorbing surfaces. In a preferred embodiment, the substrate is silicon and the moisture adsorbing mask is a silicon compound or glass. Preferably, the metal is a tungsten or other refractory metal in a gaseous form for example, tungsten hexafluoride, and the reducing gas is hydrogen.
In order to prepare a washed tube for selective vapor deposition, a bare substrate of the material to which selective deposition is to be made, for example, silicon, is placed in the tube and the tube is heated. Gas flow of the material to be deposited is introduced into the heated tube and reacts with the substrate to form a coating on the interior of the tube. The gas flow is terminated and the substrate is removed. The substrate on which selective deposition is to be performed is introduced and the selective vapor deposition process is initiated.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.